Our long-term goal is the development of a better understanding of growth and nutrition of term and preterm infants. We wish to establish nutrient requirements and to explore certain aspects of metabolism of normal infants. Few other laboratories throughout the world are similarly focused on studies of normal infants. In term and preterm infants we wish to compare methods of measuring energy balance by direct and indirect calorimetry, heart beat counting and doubly-labeled water, to explore in preterm infants the relation between energy balance, diet and body composition, and to determine the effect of feeding mode (breast or formula) on energy expenditure. We shall obtain new estimates of the requirements for phenylalanine plus tyrosine in term infants. We shall determine whether adverse interactions occur between iron, copper and zinc within the range of dietary intakes likely to be encountered by infants. Using stable isotopes of zinc, we shall investigate the major factors influencing zinc absorption. We shall determine the effect of age on fluoride absorption, excretion and retention, the effect of timing of administration of a fluoride dose (with a feeding or between feedings) on fluoride absorption and retention, and the effect of iron, we shall determine the relation between iron retention and incorporation of retained iron into erythrocytes. We shall explore factors influencing iron retention from iron-fortified infant foods in an attempt to develop strategies for preventing iron deficiency. We shall explore the infant;s requirement for biotin by balance studies and by evaluation of biotin nutritional status in relation to dietary intake of biotin.